I wanna do a time-travel story, kind of with a twist.
The whole Juubi arc goes terribly wrong, with all the non-god-tier guys getting wiped out. Hashirama yells at Tobirama to grab Naruto and 'run', and fends off Madara for time.
Tobirama then uses a time-travel jutsu that he invented, traveling back 10 years with Naruto. The two are deaged to what their age at that point in time would have been. When Naruto asks why the hell Tobirama still looks young even though he would be like 80 years old, Tobirama reveals he uses a passive regeneration formula that keeps him young, with Tsunade using an incomplete form of it. Later on, when Naruto shows off the Rasengan and Rasenshuriken, Tobirama says something about the Rasengan like 'Oh, I see the Yondaime made the third step for that incomplete jutsu I threw aside years ago.'
Then the rest of the story would basically be Tobirama being super Gary Stu and stopping Madara and Tobito right away. Because really, when the guy invented Hiraishin, Edo Tensei, and Kage Bunshin, do you really think if he has foreknowledge and isn't dead that he basically wouldn't be able to ROFLstomp the rest of the cast? (I half see him also idly mentioning at one point that Hashirama decided to build upon the results of him tinkering on Tobirama growing plants for a jutsu one afternoon).
